This invention relates generally to automatic rearview mirror systems for vehicles and more particularly to mirror systems in which the exterior mirrors are glare protected. The invention is especially adapted to electrochromic mirror systems.
Automatic rearview mirror systems have been developed in which either all solid-state electrochromic elements or electrochemichromic cells are colored by a drive signal to modify the reflectance level of the mirror, under the control of a drive circuit, in order to reduce the glare reflected to the driver. While initially applied to the interior mirror of the vehicle, such glare-protected mirrors soon found application in the exterior mirrors mounted on the driver's door and the passenger's door. It has been found convenient to provide drive signals to the two exterior mirrors and to the interior mirror, if provided, from a common drive circuit, typically located within the interior of the vehicle, in order to avoid duplication of components.
Electrochromic elements typically operate at a maximum voltage level of about 1.2 to 1.8 volts. In one type of electrochromic mirror system disclosed in commonly assigned copending patent application Ser. No. 07/643,186, filed Jan. 18, 1991, by Niall R. Lynam for an ELECTROCHROMIC MIRROR FOR VEHICLES, a heater is provided for the electrochromic element in order to provide a uniform reflectivity level across substantially the entire mirror surface irrespective of the outdoor temperature. Such a heater is operated at battery or ignition voltage levels, which are typically between 12 volts and 16 volts DC. Accordingly, when an exterior mirror is removed for repair or replacement, the mirror system is subject to inadvertent mishap. For example, the drive leads extending to the electrochromic element may be shorted together or shorted to ground. Worse, the 12-16 volt DC supply leads extending the mirror heater may inadvertently contact the exposed terminals of the electrochromic element drive lines extending from the exterior mirror housing back to the interior mirror housing. Any such mishap would likely cause damage to the drive circuit or to any mirror in the system then in use.
The combination of two or three electrochromic elements in a vehicle mirror system provides additional difficulties. The drive circuit for one mirror requires a voltage regulator that provides adequate voltage regulation with minimal excess current drain. The extra current required to drive the additional mirror(s), within the voltage range required for a full range of mirror reflectance levels, would require a conventional voltage regulator that has much greater capacity. This increase in capacity would result in excessive power dissipation in the form of heat to provide adequate voltage regulation. This difficulty is further aggravated by the necessity of proper mirror operation under conditions in which the supply voltage to the mirror system is abnormally low.
The traditional approach to driving the mirrors in an automatic rearview mirror system in which both the interior and exterior mirrors are dimmed in response to glare-producing light, is to operate all three mirrors in electrical parallel connection from the same drive circuit. This provides circuit economy and recognizes the fact that all mirrors will experience glare-producing light simultaneously. However, the glare reflected to the driver's eyes from each mirror is different because of the difference in physical relationship between the driver and each mirror. Accordingly, it is desirable to be able to decouple the operation of the mirrors in an automatic rearview mirror in order to recognize the reality of the different responses of each mirror to the same glareproducing light, when viewed from the driver.